Optical Frequency Division Multiplexing (OFDM) is a radio frequency format described, for example, by S. Hara and R. Prasad in Multicarrier Techniques for 4G Mobile Communications (Artech House, Boston, 2003). Any two subcarriers are orthogonal provided the difference in their frequency is given by m/Ts where m is an integer and Ts is symbol period. The symbol rate is the inverse of the symbol period. That is, the orthogonal subcarrier sets can be recovered with a correlator matched to the subcarrier without inter-carrier interference, in spite of strong signal spectral overlap. Radio frequency OFDM signals have some desirable properties including tolerance to dispersion and multipath interference. Optical Orthogonal Frequency Division Multiplexing (O-OFDM), where an optical tone carries an OFDM signal, has shown extreme robustness to fiber chromatic dispersion and polarization mode dispersion (PMD), and has the additional advantage of achieving high spectral efficiency using higher-order modulation enabling dynamic data rate adaptation.
Even with bandwidth efficient direct-conversion architecture in the transmitter and receiver, however, the electrical bandwidth required for fast OFDM or especially O-OFDM, is currently challenging to achieve. A 107 Gb/s O-OFDM signal would still require about 15 GHz of bandwidth. The best commercial Digital to Analogue Convert (DAC) and Analogue to Digital Convert (ADC) in a silicon integrated circuit (IC) has a bandwidth of around 6 GHz, indicating that it would be challenging to realize fast O-OFDM in a cost-effective manner.